Wireless access system using multiple modulation formats in TDD frames and method of operation

ABSTRACT

A radio frequency (RF) modem shelf for use in a fixed wireless access network comprising, a plurality of base stations capable of bidirectional time division duplex (TDD) communication with wireless access devices disposed at subscriber premises. The shelf comprises: a) a first RF modem for communicating with wireless access devices using TDD frames, each TDD frame having an uplink for, receiving data and a downlink for transmitting data; and b) a modulation controller associated with the RF modem shelf for determining an optimum modulation configuration for each of the wireless access devices communicating with the first RF modem. The controller causes the first RF modem to transmit downlink data to a first wireless access device in a first data block having a first optimum modulation configuration and to transmit downlink data to a second wireless access device in a second data block having a different second optimum modulation configuration.

CROSS-REFERNCE TO RELATED APPLICATIONS

This application is a continuation of prior U.S. patent application Ser.No. 11/702,951 filed on Feb. 6, 2007 and claims priority to: U.S. patentapplication Ser. No. 09/839,458 filed on Apr. 20, 2001, now U.S. Pat.No. 7,173,916, provisional U.S. Patent Application Ser. No. 60/262,712filed on Jan. 19, 2001 and entitled “WIRELESS COMMUNICATION SYSTEM USINGBLOCK FILTERING AND FAST EQUALIZATION-DEMODULATION AND METHOD OFOPERATION”; provisional U.S. Patent Application Ser. No. 60/262,825filed on Jan. 19, 2001 and entitled “APPARATUS AND ASSOCIATED METHOD FOROPERATING UPON DATA SIGNALS RECEIVED AT A RECEIVING STATION OF A FIXEDWIRELESS ACCESS COMMUNICATION SYSTEM”; provisional U.S. PatentApplication Ser. No. 60/262,698 filed on Jan. 19, 2001 and entitled“APPARATUS AND METHOD FOR OPERATING A SUBSCRIBER INTERFACE IN A FIXEDWIRELESS SYSTEM”; provisional U.S. Patent Application Ser. No.60/262,827 filed on Jan. 19, 2001 entitled “APPARATUS AND METHOD FORCREATING SIGNAL AND PROFILES AT A RECEIVING STATION”; provisional U.S.Patent Application Ser. No. 60/262,826 filed on Jan. 19, 2001 andentitled “SYSTEM AND METHOD FOR INTERFACE BETWEEN A SUBSCRIBER MODEM ANDSUBSCRIBER PREMISES INTERFACES”; provisional U.S. Patent ApplicationSer. No. 60/262,951 filed on Jan. 19, 2001 entitled “BACKPLANEARCHITECTURE FOR USE IN WIRELESS AND WIRELINE ACCESS SYSTEMS”;provisional U.S. Patent Application Ser. No. 60/262,824 filed on Jan.19, 2001 entitled “SYSTEM AND METHOD FOR ON-LINE INSERTION OF LINEREPLACEABLE UNITS IN WIRELESS AND WIRELINE ACCESS SYSTEMS”; provisionalU.S. Patent Application Ser. No. 60/263,101 filed on Jan. 19, 2001entitled “SYSTEM FOR COORDINATION OF TDD TRANSMISSION BURSTS WITHIN ANDBETWEEN CELLS IN A WIRELESS ACCESS SYSTEM AND METHOD OF OPERATION”;provisional U.S. Patent Application Ser. No. 60/263,097 filed on Jan.19, 2001 and entitled “REDUNDANT TELECOMMUNICATION SYSTEM USING MEMORYEQUALIZATION APPARATUS AND METHOD OF OPERATION”; provisional U.S. PatentApplication Ser. No. 60/273,579 filed Mar. 5, 2001 and entitled“WIRELESS ACCESS SYSTEM FOR ALLOCATING AND SYNCHRONIZING UPLINK ANDDOWNLINK OF TDD FRAMES AND METHOD OF OPERATION”; provisional U.S. PatentApplication Ser. No. 60/262,955 filed Jan. 19, 2001 and entitled “TDDFDD AIR INTERFACE”; provisional U.S. Patent Application Ser. No.60/262,708 filed on Jan. 19, 2001 and entitled “APPARATUS, AND ANASSOCIATED METHOD, FOR PROVIDING WLAN SERVICE IN A FIXED WIRELESS ACCESSCOMMUNICATION SYSTEM”; provisional U.S. Patent Application Ser. No.60/273,757 filed Mar. 5, 2001 and entitled “WIRELESS ACCESS SYSTEM ANDASSOCIATED METHOD USING MULTIPLE MODULATION FORMATS IN TDD FRAMESACCORDING TO SUBSCRIBER SERVICE TYPE”; provisional U.S. PatentApplication Ser. No. 60/270,378 filed Feb. 21, 2001 and entitled“APPARATUS FOR ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESSCOMMUNICATION SYSTEM”; provisional U.S. Patent Application Ser. No.60;270,385 filed Feb. 21, 2001 and entitled “APPARATUS FOR REALLOCATINGCOMMUNICATION RESOURCES TO ESTABLISH A PRIORITY CALL IN A FIXED WIRELESSACCESS COMMUNICATION SYSTEM”; and provisional U.S. Patent ApplicationSer. No. 60/270,430 filed Feb. 21, 2001 and entitled “METHOD FORESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESS COMMUNICATIONSYSTEM. Each of these applications and/or patents are herebyincorporated by reference herein in their entireties.

In addition, the subject matter disclosed in the present application isrelated to that disclosed in the following United States PatentApplications:

-   1) Ser. No. 09/713,684 filed on Nov. 15, 2000 and entitled    “SUBSCRIBER INTEGRATED ACCESS DEVICE FOR USE IN WIRELESS AND    WIRELINE ACCESS SYSTEMS”;-   2) Ser. No. 09/838,810 filed Apr. 20, 2001 and entitled “WIRELESS    COMMUNICATION SYSTEM USING BLOCK FILTERING AND FAST    EQUALIZATION-DEMODULATION AND METHOD OF OPERATION”, now U.S. Pat.    No. 7,075,967;-   3) Ser. No. 09/839,726 filed Apr. 20, 2001 and entitled “APPARATUS    AND ASSOCIATED METHOD FOR OPERATING UPON DATA SIGNALS RECEIVED AT A    RECEIVING STATION OF A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM”,    now U.S. Pat. No. 7,099,383;-   4) Ser. No. 09/839,729 filed Apr. 20, 2001 and entitled “APPARATUS    AND METHOD FOR OPERATING A SUBSCRIBER INTERFACE IN A FIXED WIRELESS    SYSTEM”, pending;-   5) Ser. No. 09/839,719 filed Apr. 20, 2001 and entitled “APPARATUS    AND METHOD FOR CREATING SIGNAL AND PROFILES AT A RECEIVING STATION”,    now U.S. Pat. No. 6,947,477;-   6) Ser. No. 09/838,910 filed Apr. 20, 2001 and entitled “SYSTEM AND    METHOD FOR INTERFACE BETWEEN A SUBSCRIBER MODEM AND SUBSCRIBER    PREMISES INTERFACES”, now U.S. Pat. No. 6,564,051;-   7) Ser. No. 09/839,509 filed Apr. 20, 2001 and entitled “BACKPLANE    ARCHITECTURE FOR USE IN WIRELESS AND WIRELINE ACCESS SYSTEMS”,    pending;-   8) Ser. No. 09/839,514 filed Apr. 20, 2001 and entitled “SYSTEM AND    METHOD FOR ON-LINE INSERTION OF LINE REPLACEABLE UNITS IN WIRELESS    AND WIRELINE ACCESS SYSTEMS”, now U.S. Pat. No. 7,069,047;-   9) Ser. No. 09/839,512 filed Apr. 20, 2001 and entitled “SYSTEM FOR    COORDINATION OF TDD TRANSMISSION BURSTS WITHIN AND BETWEEN CELLS IN    A WIRELESS ACCESS SYSTEM AND METHOD OF OPERATION”, now U.S. Pat. No.    6,804,527;-   10) Ser. No. 09/839,259 filed Apr. 20, 2001 and entitled “REDUNDANT    TELECOMMUNICAION SYSTEM USING MEMORY EQUALIZATION APPARATUS AND    METHOD OF OPERATION”, now U.S. Pat. No. 7,065,098;-   11) Ser. No. 09/839,457 filed Apr. 20, 2001 and entitled “WIRELESS    ACCESS SYSTEM FOR ALLOCATING AND SYNCHRONIZING UPLINK AND DOWNLINK    OF TDD FRAMES AND METHOD OF OPERATION”, now U.S. Pat. No. 7,002,929;-   12) Ser. No. 09/839,075 filed Apr. 20, 2001 and entitled “TDD FDD    AIR INTERFACE”, now U.S. Pat. No. 6,859,655;-   13) Ser. No. 09/839,499 filed Apr. 20, 2001 and entitled “APPARATUS,    AND AN ASSOCIATED METHOD, FOR PROVIDING WLAN SERVICE IN A FIXED    WIRELESS ACCESS COMMUNICATION SYSTEM”, pending;-   14) Ser. No. 09/839,456 filed Apr. 20, 2001 and entitled “WIRELESS    ACCESS SYSTEM AND ASSOCIATED METHOD USING MULTIPLE MODULATION    FORMATS IN TDD FRAMES ACCORDING TO SUBSCRIBER SERVICE TYPE”, now    U.S. Pat. No. 6,891,810;-   15) Ser. No. 09/838,924 filed Apr. 20, 2001 and entitled “APPARATUS    FOR ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESS    COMMUNICATION SYSTEM”, pending;-   16) Ser. No. 09/839,727 filed Apr. 20, 2001 and entitled “APPARATUS    FOR REALLOCATING COMMUNICATION RESOURCES TO ESTABLISH A PRIORITY    CALL IN A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM”, now U.S. Pat.    No. 7,031,738;-   17) Ser. No. 09/839,734 filed Apr. 20, 2001 and entitled “METHOD FOR    ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESS    COMMUNICATION SYSTEM”, now U.S. Pat. No. 7,035,241; and-   18) Ser. No. 09/839,513 filed Apr. 20, 2001 and entitled “SYSTEM AND    METHOD FOR PROVIDING AN IMPROVED COMMON CONTROL BUS FOR USE IN    ON-LINE INSERTION OF LINE REPLACEABLE UNITS IN WIRELESS AND WIRELINE    ACCESS SYSTEMS”, now U.S. Pat. No. 6,925,516.

The above applications are commonly assigned to the assignee of thepresent application. The disclosures of these related patentapplications may share common subject matter and figures and are herebyincorporated by herein in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to wireless accesssystems and, more specifically, to a burst packet transmission mediaaccess system for use in a fixed wireless access network.

BACKGROUND OF THE INVENTION

Telecommunications access systems provide for voice, data, andmultimedia transport and control between the central office (CO) of thetelecommunications service provider and the subscriber (customer)premises. Prior to the mid-1970s, the subscriber was provided phonelines (e.g., voice frequency (VF) pairs) directly from the Class 5switching equipment located in the central office of the telephonecompany. In the late 1970s, digital loop carrier (DLC) equipment wasadded to the telecommunications access architecture. The DLC equipmentprovided an analog phone interface, voice CODEC, digital datamultiplexing, transmission interface, and control and alarm remotelyfrom the central office to cabinets located within business andresidential locations for approximately 100 to 2000 phone lineinterfaces. This distributed access architecture greatly reduced linelengths to the subscriber and resulted in significant savings in bothwire installation and maintenance. The reduced line lengths alsoimproved communication performance on the line provided to thesubscriber.

By the late 1980s, the limitations of data modem connections over voicefrequency (VF) pairs were becoming obvious to both subscribers andtelecommunications service providers. ISDN (Integrated Services DigitalNetwork) was introduced to provide universal 128 kbps service in theaccess network. The subscriber interface is based on 64 kbpsdigitization of the VF pair for digital multiplexing into high speeddigital transmission streams (e.g., T1/T3 lines in North America, E1/E3lines in Europe). ISDN was a logical extension of the digital networkthat had evolved throughout the 1980s. The rollout of ISDN in Europe washighly successful. However, the rollout in the United States was notsuccessful, due in part to artificially high tariff costs which greatlyinhibited the acceptance of ISDN.

More recently, the explosion of the Internet and disregulation of thetelecommunications industry have brought about a broadband revolutioncharacterized by greatly increased demands for both voice and dataservices and greatly reduced costs due to technological innovation andintense competition in the telecommunications marketplace. To meet thesedemands, high speed DSL (digital subscriber line) modems and cablemodems have been developed and introduced. The DLC architecture wasextended to provide remote distributed deployment at the neighborhoodcabinet level using DSL access multiplexer (DSLAM) equipment. Theincreased data rates provided to the subscriber resulted in upgradeDLC/DSLAM transmission interfaces from T1/E1 interfaces (1.5/2.0 Mbps)to high speed DS3 and OC3 interfaces. In a similar fashion, the entiretelecommunications network backbone has undergone and is undergoingcontinuous upgrade to wideband optical transmission and switchingequipment.

Similarly, wireless access systems have been developed and deployed toprovide broadband access to both commercial and residential subscriberpremises. Initially, the market for wireless access systems was drivenby rural radiotelephony deployed solely to meet the universal servicerequirements imposed by government (i.e., the local telephone company isrequired to serve all subscribers regardless of the cost to installservice). The cost of providing a wired connection to a small percentageof rural subscribers was high enough to justify the development andexpense of small-capacity wireless local loop (WLL) systems.

Deregulation of the local telephone market in the United States (e.g.,Telecommunications Act of 1996) and in other countries shifted the focusof fixed wireless access (FWA) systems deployment from rural access tocompetitive local access in more urbanized areas. In addition, the ageand inaccessibility of much of the older wired telephone infrastructuremakes FWA systems a cost-effective alternative to installing new, wiredinfrastructure, Also, it is more economically feasible to install FWAsystems in developing countries where the market penetration is limited(i.e., the number and density of users who can afford to pay forservices is limited to small percent of the population) and the rolloutof wired infrastructure cannot be performed profitably. In either case,broad acceptance of FWA systems requires that the voice and data qualityof FWA systems must meet or exceed the performance of wiredinfrastructure.

Wireless access systems must address a number of unique operational andtechnical issues including:

1) Relatively high bit error rates (HER) compared to wire line oroptical systems; and

2) Transparent operation with network protocols and protocol timeconstraints for the following protocols:

a) ATM;

b) Class 5 switch interfaces (domestic GR-303 and international V5.2);

c) TCP/IP with quality-of-service QoS for voice over IP (VoIP) (i.e.,RTP) and other H.323 media services;

-   -   d) Distribution of synchronization of network time out to the        sub'scribers;

3) increased use of voice, video and/or media compression andconcentration of active traffic over the air interface to conservebandwidth;

4) Switching and routing within the access system to distribute signalsfrom the central office to multiple remote cell sites containingmultiple cell sectors and one or more frequencies of operation persector; and

5) Remote support and debugging of the subscriber equipment, includingremote software upgrade and provisioning.

Unlike physical optical or wire systems that operate at bit error rates(BER) of 10⁻¹¹, wireless access systems have time varying channels thattypically provide bit error rates of 10⁻³ to 10⁻⁶. The wireless physical(PHY) layer interface and the media access control (MAC) layer interfacemust provide modulation, error correction and ARQ (automatic request forretransmission) protocol that can detect and, where required, correct orretransmit corrupted data so that the interfaces at the network and atthe subscriber site operate at wire line bit error rates.

The wide range of equipment and technology capable of providing eitherwireline (i.e., cable, DSL, optical) broadband access or wirelessbroadband access has allowed service providers to match the needs of asubscriber with a suitable broadband access solution. However, in manyareas, the cost of cable modem or DSL service is high. Additionally,data rates may be slow or coverage incomplete due to line lengths. Inthese areas and in areas where the high cost of replacing old telephoneequipment or the low density of subscribers makes it economicallyunfeasible to introduce either DSL or cable modem broadband access,fixed wireless broadband systems offer a viable alternative. Fixedwireless broadband systems use a group of transceiver base stations tocover a region in the same manner as the base stations of a cellularphone system. The base stations of a fixed wireless broadband systemtransmit forward channel (i.e., downstream) signals in directed beams tofixed location antennas attached to the residences or offices ofsubscribers. The base stations also receive reverse channel (i.e.,upstream) signals transmitted by the broadband access equipment of thesubscriber.

Media access control (MAC) protocols refer to techniques that increaseutilization of two-way communication channel resources by subscribersthat use the channel-resources. The MAC layer may use a number ofpossible configurations to allow multiple access. These configurationsinclude:

1. FDMA—frequency division multiple access. In a FDMA system,subscribers use separate frequency channels on a permanent or demandaccess basis.

2. TDMA—time division multiple access. In a TDMA system, subscribersshare a frequency channel but allocate spans of time to different users.

3. CDMA—code division multiple access. In a CDMA system, subscribersshare a frequency but use a set of orthogonal codes to allow multipleaccess.

4. SDMA—space division multiple access—In a SDMA system, subscribersshare a frequency but one or more physical channels are formed usingantenna beam forming techniques.

5. PDMA—polarization division multiple access—In a PDMA system,subscribers share a frequency but change polarization of the antenna.

Each of these MAC techniques makes use of a fundamental degree offreedom (physical property) of a communications channel. In practice,combinations of these degrees of freedom are often used. As an example,cellular systems use a combination of FDMA and either TDMA or CDMA tosupport a number of users in a cell.

To provide a subscriber with bi-directional (two-way) communication in ashared media, such as a coaxial cable, a multi-mode fiber (optical), oran RF radio channel, some type of duplexing technique must beimplemented. Duplexing techniques include frequency division duplexing(FDD) and time division duplexing (TDD). In FDD, a first channel(frequency) is used for transmission and a second channel (frequency) isused for reception. To avoid physical interference between the transmitand receive channels, the frequencies must have a separation know as theduplex spacing. In TDD, a single channel is used for transmission andreception and specific periods of time (i.e., slots) are allocated fortransmission and other specific periods of time are allocated forreception.

Finally, a method of coordinating the use of bandwidth must beestablished. There are two fundamental methods: distributed control andcentralized control. In distributed control, subscribers have a sharedcapability with or without a method to establish priority. An example ofthis is CSMA (carrier sense multiple access) used in IEEE802.3 Ethernetand IEEE 802.11 Wireless LAN. In centralized control, subscribers areallowed access under the control of a master controller. Cellularsystems, such as IS-95, IS-136, and GSM, are typical examples. Access isgranted using forms of polling and reservation (based on polled ordemand access contention).

A number of references and overviews of demand access are availableincluding the following:

-   1. Sklar, Bernard. “Digital Communications Fundamentals and    Applications,” Prentice Hall, Englewood Cliffs, N.J., 1988. Chapter    9.-   2. Rappaport, Theodore. “Wireless Communications, Principles and    Practice,” Prentice Hall, Upper Saddle River, N.J., 1996. Chapter 8.-   3. TR101-173V1.1. “Broadband Radio Access Networks, Inventory of    Broadband Radio Technologies and Techniques,” ETSI, 1998. Chapter 7.

The foregoing references are hereby incorporated by reference into thepresent disclosure as if fully set forth herein.

In 1971, the University of Hawaii began operation of a random accessshared channel ALOHA TDD system. The lack of channel coordinationresulted in poor utilization of the channel. This lead to theintroduction of time slots (slotted Aloha) that set a level ofcoordination between the subscribers that doubled the channelthroughput. Finally, the researchers introduced the concept of a centralcontroller and the use of reservations (reservation Aloha). Reservationtechniques made it possible to make trade-offs between throughput andlatency.

This work was fundamental to the development of media access control(MAC) techniques for dynamic random access and the use of ARQ (automaticrequest for retransmission) to retransmit erroneous packets. While thework at the University of Hawaii explored the fundamentals of bursttransmission and random access, the work did not introduce the conceptof a frame and/or super-frame structure to the TDD/TDMA accesstechniques. One of the more sophisticated systems developed in the 1970sand in current use is Joint Tactical Information Distribution System(JTIDS). This system was based on the joint use of TDMA and timeduplexing over frequency-hopping spread-spectrum channels. This was theculmination of research to allow flexible allocation of bandwidth to alarge group of users. The key aspect of the JTIDS system was theintroduction of dynamic allocation of bandwidth resources and explicitvariable symmetry (downlink vs. uplink bandwidth) in the link.

IEEE 802.11 Wireless LAN equipment provides for a centrally coordinatedTDD system that does not have a specific frame or slotting structure.IEEE 802.11 did introduce the concept of variable modulation andspreading inherent in the structure of the transmission bursts. Asignificant improvement was incorporated in U.S. Pat. No. 6,052,408,entitled “Cellular Communications System with Dynamically Modified DataTransmission Parameters.” This patent introduced specific burst packettransmission formats that provide for adaptive modulation, transmitpower, and antenna beam forming and an associated method of determiningthe highest data rate for a defined error rate floor for the linkbetween the base station and a plurality of subscribers assigned to thatbase station. With the exception of variable spreading military systemsand NASA space communication systems, this was one of the firstcommercial patents that address variable transmission parameters toincrease system throughput.

Another example of TDD systems are digital cordless phones, alsoreferred to as low-tier PCS systems. The Personal Access Communications(PAC) system and Digital European Cordless Telephone (DECT, as specifiedby ETSI document EN 300-175-3) are two examples of these systems.Digital cordless phones met with limited success for their intended useas pico-cellular fixed access products. The systems were subsequentlymodified and repackaged for wireless local loop (WLL) applications withextended range using increased transmission (TX) power and greaterantenna gain.

These TDD/TDMA systems use fixed symmetry and bandwidth between theuplink and the downlink. The TDD frame consists of a fixed set of timeslots for the uplink and the downlink. The modulation index (or type)and the forward error correction (FEC) format for all data transmissionsare fixed in these systems. These systems did not include methods forcoordinating TDD bursts between systems. This resulted in inefficientuse of spectrum in the frequency planning of cells.

While DECT and PAC systems based on fixed frames with fixed andsymmetric allocation of time slots (or bandwidth) provides excellentlatency and low jitter, and can support time bounded services, such asvoice and N×64 Kbps video, these systems do not provide efficient use ofthe spectrum when asymmetric data services are used. This has lead toresearch and development of packet based TDD systems based on Internetprotocol (IP) or asynchronous transfer mode (ATM), with dynamicallocation of TDD time slots and the uplink-downlink bandwidth, combinedwith efficient algorithms to address both best efforts and real-timelow-latency service for converged media access (data and multi media).

One example of a TDD system with dynamic slot and bandwidth assignmentis the ETSI HYPERLAN II specification based on the Dynamic SlotAssignment algorithm described in “Wireless ATM: Performance Evaluationof a DSA++ MAC Protocol with Fast Collision resolution by ProbingAlgorithm,” D. Petras and A. Kramling, International Journal of WirelessInformation Networks, Vol. 4, No. 4, 1997. This system allows bothcontention-based and contention-free access to the physical TDD channelslots. This system also introduced the broadcast of resource allocationat the start of every frame by the base station controller. Otherwireless standards, including IEEE 802.16 wireless metropolitan networkstandards, use this combination of an allocation MAP of the uplink anddownlink at the start of the dynamic TDD frame to set resource use forthe next TDD frame.

An further improvement to this TDD system was described in “MultipleAccess Control Protocols for wireless ATM: Problem Definition and DesignObjectives,” O. Kubbar and H. Mouftah, IEEE Communications, November1997, pp. 93-99. This system expanded on the packet reservation multipleaccess (PRMA) method developed in 1989 at Rutgers University WINLAB forATM and IP based transport [see “Packet Reservation Multiple Access forLocal Wireless Communications,” Goodman et al., IEEE Transaction onCommunications, Vol. 37, No. 8, pp. 885-890]. Like PRMA, this systemlogically arranged all the downlink transmissions in the start of afixed duration TDD frame and all uplink transmissions at the end of theTDD frame. This eliminated the inefficiencies in the DCA++ Hyperlan IIprotocol. Adaptive allocation of uplink and downlink bandwidth issupported. The system provided for fixed, random, and demand assignmentmechanisms. Priority is given to quality of service (QoS) applicationswith resources being removed from best efforts demand access users asrequired.

The above-described prior art concern the allocation of services in anindividual sector of a cell. A cell may consist of M sectors, whereineach sector generally covers a 360/M degree arc around the cell site.Each sector serves N_(m) subscribers, where m=1 to M. These referencesdid not expressly provide protocol mechanisms or rules for the operationof a given system.

U.S. Pat. No. 6,016,311 expressly addresses one possible implementationto the TDD bandwidth allocation problem. The system describedcontinuously measures and adapts the bandwidth requirements based on theevaluation of the average bandwidth required by all the subscribers in acell and the number of times bandwidth is denied to the subscribers.Changes to the bandwidth allocation are applied based on a set of rulesdescribed in U.S. Pat. No. 6,016,311. While measurements of multiplesectors are performed and recorded at a central base station controller,no global coordination of bandwidth allocation of multiple sectors in acell or across multiple cells is provided.

Thus, the prior art does not address two very important factors inallocation of bandwidth. First, bandwidth allocation must contemplatestringent bandwidth availability requirements for specific groups ofservices based on planning of the network. For example, considerlife-line toll quality voice service. Toll quality voice requires that asystem guarantee a specific maximum blocking probability for all voiceusers based on peak busy hour call usage. A description of voice trafficplanning is provided in “Digital Telephony—2^(nd) Edition,” by J.Bellamy, John Wiley and Sons, New York, N.Y., 1990. If a TDD system isdesigned to meet life-line voice requirements, the allocation protocolmust be able to rapidly (i.e., less than 100 msec) reallocate bandwidthresources up to the capacity necessary to meet the call blockingrequirements. Another service group example is a guaranteed servicelevel agreements (SLA). Again, bandwidth must be rapidly restored tomeet the SLA conditions. More generally, one may consider G possibleservice groups having a set of weighted priority level and associatedminimum and maximum levels. The weighted priority levels and minimum andmaximum levels may be used to bound the bandwidth dynamics of the TDDbandwidth allocation. Minimum levels set a floor for bandwidthallocation and maximum levels set a ceiling. Then averaging can beapplied.

Second, the TDD bandwidth allocation must consider adjacent andco-channel interference from both modems and sectors within a cell andbetween cells. Cell planning tools can be used to establish therelationships for interference. For systems that operate below 10 GHz,antennas and antenna placement at a cell site will not provide adequatesignal isolation. These co-channel interference issues are welldocumented in “Frequency Reuse and System Deployment in Local MultipointDistribution Service,” by V. Roman, IEEE Personal Communications,December 1999, pp. 20 to 27.

Therefore, there is a need in the art for a fixed wireless accessnetwork that maximizes spectral efficiency between the base stations ofthe fixed wireless access network and the subscriber access deviceslocated at the subscriber premises. In particular, there is a need for afixed wireless access network that implements an air interface thatminimizes uplink and downlink interference between different sectorswithin the same base station cell site. There also is a need for a fixedwireless access network that implements an air interface that minimizesuplink and downlink interference between different cell sites within thefixed wireless access network. More particularly, there is a need in theart for a fixed wireless that efficiently allocates bandwidth toindividual subscribers according to dynamically changing applicationsused by the individual subscribers.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is aprimary object of the present invention to provide an improved airinterface system for use in a fixed wireless access network thatmaximizes usage of the available bandwidth in a cell site. The systemuses multiple modulation groups in the air interface to transmit datato, and to receive data from, subscriber access devices in each cellsite and/or each sector within a cell site. Each subscriber accessdevice is added to a modulation group according to the modulationformat, FEC code, and/or antenna beam (spatial component) parameter thatgives the highest data throughput to each subscriber access devicewithin acceptable error limitations. Subscriber access devices thatreceive a comparatively noisy signal may receive a downlink signal inBPSK format with a high level of FEC protection. Subscriber accessdevices that receive a relatively noise-free signal may receive adownlink signal in 16 QAM format with a relatively low level of FECprotection. Subscriber access devices using the same modulation formatand FEC coding are grouped together in the same modulation groups in theuplink and the downlink. Furthermore, if two or more subscriber accessdevices lie in the same direction away from a base station, thosesubscriber access devices may also be grouped together in the samemodulation group according to antenna beam forming (i.e., spatialmodulation) parameters of the base station. Similar modulation groupingsare used in the uplink from the subscriber access devices to thetransceiver base station.

Thus, it is a primary object of the present invention to provide, aradio frequency (RF) modem shelf for use in a fixed wireless accessnetwork comprising a plurality of base stations capable of bidirectionaltime division duplex (TDD) communication with wireless access devicesdisposed at a plurality of subscriber premises. According to anadvantageous embodiment of the present invention, the radio frequency(RF) modem shelf comprises: a) a first RF modem capable of communicatingwith a plurality of the wireless access devices using TDD frames, eachTDD frame having an uplink for receiving data and a downlink fortransmitting data; b) a modulation controller associated with the RFmodem shelf capable of determining an optimum modulation configurationfor each of the plurality of wireless access devices communicating withthe first RF modem, wherein the modulation controller causes the firstRF modem to transmit downlink data to a first wireless access device ina first data block having a first optimum modulation configuration andto transmit downlink data to a second wireless access device in a seconddata block having a different second optimum modulation configuration

According to one embodiment of the present invention, the modulationcontroller determines the first and second optimum modulationconfigurations based on channel conditions associated with channels usedto communicate with the first and second wireless access devices.

According to another embodiment of the present invention, the firstmodulation configuration comprises a first modulation format and thesecond modulation configuration comprises a different second modulationformat.

According to still another embodiment of the present invention, thesecond modulation format is more complex than the first modulationformat if channel conditions associated with a first channel used tocommunicate with the first wireless access device are noisier thanchannel conditions associated with a second channel used to communicatewith the second wireless access device.

According to yet another embodiment of the present invention, the firstand second modulation formats comprise one of binary phase shift keying(BPSK), quadrature phase shift keying (QPSK), and 16 quadratureamplitude modulation (QAM).

According to a further embodiment of the present invention, the firstmodulation configuration comprises a first forward error correction codelevel and the second modulation configuration comprises a differentsecond forward error correction code level.

According to a still further embodiment of the present invention, thefirst error correction code level is more complex than the second errorcorrection code level if channel conditions associated with a firstchannel used to communicate with the first wireless access device arenoisier than channel conditions associated with a second channel used tocommunicate with the second wireless access device.

According to a yet further embodiment of the present invention, thefirst modulation configuration comprises a first physical beam formingtechnique and the second modulation configuration comprises a differentsecond physical beam forming technique.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention so that those skilled in the art maybetter understand the detailed description of the invention thatfollows. Additional features and advantages of the invention will bedescribed hereinafter that form the subject of the claims of theinvention. Those skilled in the art should appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. Those skilled in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the invention in its broadest form.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions or certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, wherein likenumbers designate like objects, and in which:

FIG. 1 illustrates an exemplary fixed wireless access network accordingto one embodiment of the present invention;

FIG. 2 illustrates in greater detail an alternate view of selectedportions of the exemplary fixed wireless access network according to oneembodiment of the present invention;

FIG. 3 illustrates an exemplary time division duplex (TDD) time divisionmultiple access (TDMA) frame according to one embodiment of the presentinvention;

FIG. 4 illustrates the timing recovery and distribution circuitry in anexemplary RF modem shelf according to one embodiment of the presentinvention;

FIG. 5A illustrates an exemplary time division duplex (TDD) framesaccording to one embodiment of the present invention;

FIG. 5B illustrates an exemplary transmission burst containing a singleFEC block according to one embodiment of the present invention;

FIG. 5C illustrates an exemplary transmission burst containing multipleFEC blocks according to one embodiment of the present invention;

FIG. 6 is a flow diagram illustrating the adaptive modification of theuplink and downlink bandwidth in the air interface in wireless accessnetwork according to one embodiment of the present invention;

FIG. 7 is a flow diagram illustrating the adaptive assignment ofselected link parameters, such as modulation format, forward errorcorrection (FEC) codes, and antenna beam forming, to the uplink anddownlink channels used by each subscriber in the exemplary wirelessaccess network according to one embodiment of the present invention; and

FIG. 8 is a flow diagram illustrating the adaptive assignment ofselected link parameters to the different service connections used byeach subscriber in the wireless access network according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 8, discussed below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the present invention may beimplemented in any suitably arranged wireless access system.

FIG. 1 illustrates exemplary fixed wireless access network 100 accordingto one embodiment of the present invention. Fixed wireless network 100comprises a plurality of transceiver base stations, including exemplarytransceiver base station 110: that transmit forward channel (i.e.,downlink or downstream) broadband signals to a plurality of subscriberpremises, including exemplary subscriber premises 121, 122 and 123, andreceive reverse channel (i.e., uplink or upstream) broadband signalsfrom the plurality of subscriber premises. Subscriber premises 121-123transmit and receive via fixed, externally-mounted antennas 131-133,respectively. Subscriber premises 121-123 may comprise many differenttypes of residential and commercial buildings, including single family,homes, multi-tenant office, small business enterprises (SBE), mediumbusiness enterprises (MBE), and so-called “SOHO” (small office/homeoffice) premises.

The transceiver base stations, including transceiver base station 110,receive the forward channel (i.e., downlink) signals from externalnetwork 150 and transmit the reverse channel (i.e., uplink) signals toexternal network 150. External network 150 may be, for example, thepublic switched telephone network (PSTN) or one or more data networks,including the Internet or proprietary Internet protocol (IP) wide areanetworks (WANs) and local area networks (LANs). Exemplary transceiverbase station 110 is coupled to RF modem shelf 140, which, among otherthings, up-converts baseband data traffic received from external network150 to RF signals transmitted in the forward channel to subscriberpremises 121-123. RF modem shelf 140 also down-converts RF signalsreceived in the reverse channel from subscriber premises 121-123 tobaseband data traffic that is transmitted to external network 150.

RF modem shelf 140 comprises a plurality of RF modems capable ofmodulating (i.e., up-converting) the baseband data traffic anddemodulating (i.e., down-converting) the reverse channel RF signals. Inan exemplary embodiment of the present invention, each of thetransceiver base stations covers a cell site area that is divided into aplurality of sectors. In an advantageous embodiment of the presentinvention, each of the RF modems in RF modem shelf 140 may be assignedto modulate and demodulate signals in a particular sector of each cellsite. By way of example, the cell site associated with transceiver basestation 110 may be partitioned into six sectors and RF modem shelf 140may comprise six primary RF modems (and, optionally, a seventh spare RFmodem), each of which is assigned to one of the six sectors in the cellsite of transceiver base station 110. In another advantageous embodimentof the present invention, each RF modem in RF modem shelf 140 comprisestwo or more RF modem transceivers which may be assigned to at least oneof the sectors in the cell site. For example, the cell site associatedwith transceiver base station 110 may be partitioned into six sectorsand RF modem shelf 140 may comprise twelve RF transceivers that areassigned in pairs to each one of the six sectors. The RF modems in eachRF modem pair may alternate modulating and demodulating the downlink anduplink signals in each sector.

RF modem shelf 140 is located proximate transceiver base station 110 inorder to minimize RF losses in communication line 169. RF modem shelf140 may receive the baseband data traffic from external network 150 andtransmit the baseband data traffic to external network 150 via a numberof different paths. In one embodiment of the present invention, RF modemshelf 140 may transmit baseband data traffic to, and receive basebanddata traffic from, external network 150 through central office facility160 via communication lines 166 and 167. In such an embodiment,communication line 167 may be a link in a publicly owned or privatelyowned backhaul network. In another embodiment of the present invention,RF modem shelf 140 may transmit baseband data traffic to, and receivebaseband data traffic from, external network 150 directly viacommunication line 168 thereby bypassing central office facility 160.

Central office facility 160 comprises access processor shelf 165. Accessprocessor shelf 165 provides a termination of data traffic for one ormore RF modem shelves, such as RF modem shelf 140. Access processorshelf 165 also provides termination to the network switched circuitinterfaces and/or data packet interfaces of external network 150. One ofthe principal functions of access processor shelf 165 is to concentratedata traffic as the data traffic is received from external network 150and is transferred to RF modem shelf 140. Access processor shelf 165provides data and traffic processing of the physical layer interfaces,protocol conversion, protocol management, and programmable voice anddata compression.

FIG. 2 illustrates in greater detail an alternate view of selectedportions of exemplary fixed wireless access network 100 according to oneembodiment of the present invention. FIG. 2 depicts additionaltransceiver base stations, including exemplary transceiver base stations110A through 110F, central office facilities 160A and 160B, and remoteRF modem shelves 140A through 140D. Central office facilities 160A and160B comprise internal RF moderns similar to RF modem shelves 140Athrough 140D. Transceiver base stations 110A, 110B, and 110C aredisposed in cells sites 201, and 203, respectively. In the exemplaryembodiment, cell sites 201-203 (shown in dotted lines) are partitionedinto four sectors each. In alternate embodiments, sites 201, 202, and203 may be partitioned into a different number of sectors, such as sixsectors, for example.

As in FIG. 1, RF modem shelves 140A-140D and the internal RF modems ofcentral office facilities 160A and 160E transmit baseband data trafficto, and receive baseband data traffic from, access processors in centraloffice facilities 160A and 160B of the PSTN. RF modem shelves 140A-140Dand the internal RF modems of central office facilities 160A and 160Balso up-convert incoming baseband data traffic to RF signals transmittedin the forward (downlink) channel to the subscriber premises anddown-convert incoming RF signals received in the reverse (uplink)channel to baseband data traffic that is transmitted via a backhaulnetwork to external network 150.

Baseband data traffic may be transmitted from remote RF modem shelves140A-140D to central office facilities 160A and 160B by a wirelessbackhaul network or by a wireline backhaul network, or both. As shown inFIG. 2, baseband data traffic is carried between central office facility160A and remote RF modem 140A by a wireline backhaul network, namelywireline 161, which may be, for example, a DS3 line or one to N T1lines. A local multipoint distribution service (LMDS) wireless backhaulnetwork carries baseband data traffic between central office facilities160A and 160B and remote RF modem shelves 140B, 140C, and 140D. In aLMDS wireless backhaul network, baseband data traffic being sent toremote RF modem shelves 140B, 140C, and 140D is transmitted by microwavefrom microwave antennas mounted on transceiver base stations 110A, 110C,and 110F to microwave antennas mounted on transceiver base stations110E, 110D, and 110E. Baseband data traffic being sent from remote RFmodem shelves 140B, 140C, and 40D is transmitted by microwave in thereverse direction (i.e., from transceiver base stations 110B, 110D, and110E to transceiver base stations 110A, 110C, and 110F).

At each of transceiver base stations 110B, 110D, and 110E, downlink datatraffic from central office facilities 160A and 160B is down-convertedfrom microwave frequencies to baseband signals before being up-convertedagain for transmission to subscriber premises within each cell site.Uplink data traffic received from the subscriber premises isdown-converted to baseband signals before being up-converted tomicrowave frequencies for transmission back to central office facilities160A and 160B.

Generally, there is an asymmetry of data usage in the downlink and theuplink. This asymmetry is typically greater than 4:1 (downlink:uplink).Taking into account the factors of data asymmetry, channel propagation,and available spectrum, an advantageous embodiment of the presentinvention adopts a flexible approach in which the physical (PHY) layerand the media access (MAC) layer are based on the use of time divisionduplex (TDD) time division multiple access (TDMA). TDD operations sharea single RF channel between a transceiver base station and a subscriberpremises and use a series of frames to allocate resources between eachuser uplink and downlink. A great advantage of TDD operation is theability to dynamically allocate the portions of a frame allocatedbetween the downlink and the uplink. This results in an increasedefficiency of operation relative to frequency division duplex (FDD)techniques. TDD operations typically may achieve a forty to sixtypercent advantage in spectral efficiency over FDD operations undertypical conditions. Given the short duration of the transmit and receivetime slots relative to changes in the channel, TDD operations alsopermit open loop power control, switched diversity techniques, andfeedforward and cyclo-stationary equalization techniques that reducesystem cost and increase system throughput.

To aid with periodic functions in the system, TDD frames are groupedinto superframes (approximately 10 to 20 milliseconds). The superframesare further grouped into hyperframes (approximately 250 to 1000milliseconds). This provides a coordinated timing reference tosubscriber integrated access devices in the system. FIG. 3 illustratesan exemplary time division duplex (TDD) time division multiple access(TDMA) framing hierarchy according to one embodiment of the presentinvention. At the highest level, the TDD-TDMA framing hierarchycomprises hyperframe 310, which is X milliseconds (msec.) in length(e.g., 250 msec.<X<1000 msec.). Hyperframe 310 comprises N superframes,including exemplary superframes 311-316. Each of superframes 311-316 is20 milliseconds in duration.

Superframe 313 is illustrated in greater detail. Superframe 313comprises ten (10) TDD frames, including exemplary TDD frames 321-324,which are labeled TDD Frame 0, TDD Frame 1, TDD Frame 2, and TDD Frame9, respectively. In the exemplary embodiment, each TDD frame is 2milliseconds in duration. A TDD transmission frame is based on a fixedperiod of time during which access to the channel is controlled by thetransceiver base station.

Exemplary TDD frame 321 is illustrated in greater detail. TDD frame 321comprises a downlink portion (i.e, base station to subscribertransmission) and an uplink portion (i.e., subscriber to base stationtransmission). In particular, TDD frame 321 comprises:

Frame header 330—Frame header 330 is a broadcast message thatsynchronizes the start of frame and contains access control informationon how the remainder of TDD frame 321 is configured. The modulationformat of frame header 330 is chosen so that all subscribers in a sectorof the transceiver base station can receive frame header 330. Generally,this means that frame header 330 is transmitted in a very low complexitymodulation format, such as binary phase shift keying (BPSK or 2-BPSK),or perhaps quadrature phase shift keying (QPSK or 4-BPSK).

D downlink slots—The D downlink slots, including exemplary downlinkslots 341-343, contain transceiver base station-to-subscribertransmissions of user traffic and/or control signals. The modulationformat of each slot is optimized for maximum possible data transmissionrates. Downlink slots may be grouped in blocks to form modulation groupsas shown in FIG. 5A. Subscribers who receive data using the samemodulation format (or modulation index) and the same forward errorcorrection (FEC) codes are grouped together in the same modulationgroup. In some embodiment of the present invention, two or moremodulation groups may have the same modulation format and FEC codes. Inalternate embodiments of the present invention, downlink slots may begrouped in blocks based on physical beam forming; rather than onmodulation format and FEC codes. For example, a transceiver base stationmay transmit data to several subscribers that are directionally alongthe same antenna beam in consecutive bursts. In still other embodimentsof the present invention, downlink slots may be grouped in blocks basedon any combination of two or more of: 1) physical beam forming, 2)modulation format, and 3) FEC codes. For the purpose of simplicity, theterm “modulation group” shall be used hereafter to refer to a group ofdownlink slots that are transmitted to one or more subscribers using acommon scheme consisting of one or more of modulation format, FEC codes,and physical beam forming.

U uplink slots—The U uplink slots, including exemplary uplink slots361-363, contain subscriber-to-transceiver base station transmissions ofuser traffic and/or control signals. Again, the modulation format(modulation index) is optimized for maximum possible data transmissionrates. Generally, the modulation format and FEC codes in the uplinkslots are less complex than in the downlink slots. This moves complexityto the receivers in the base stations and lowers the cost and complexityof the subscriber access device. Uplink slots maybe grouped in blocks toform sub-burst groups as shown in FIG. 5A. Subscribers who transmit datausing the same modulation format (or modulation index) and the sameforward error correction (FEC) codes are grouped together in the samesub-burst group. In some embodiments of the present invention, two ormore sub-burst groups may have the same modulation format and FEC codes.In other embodiments of the present invention, uplink slots may begrouped in blocks based on physical beam forming, rather than onmodulation format and FEC codes. In other embodiments, uplink slots maybe grouped in blocks based on any combination of two or more of: 1)physical beam forming, 2) modulation format, and 3) FEC codes. For thepurpose of simplicity, the term “sub-burst group” shall be usedhereafter to refer to a group of uplink slots that are transmitted toone or more subscribers using a common scheme consisting of one or moreof modulation format, FEC codes, and physical beam forming.

Contention slots 360—Contention slots 360 precede the U uplink slots andcomprise a small number of subscriber-to-base transmissions that handleinitial requests for service. A fixed format length and a singlemodulation format suitable for all subscriber access devices are usedduring contention slots 360. Generally, this means that contention slots360 are transmitted in a very low complexity modulation format, such asbinary phase shift keying (BPSK or 2-BPSK), or perhaps quadrature phaseshift keying (QPSK or 4-BPSK). Collisions (more than one user on a timeslot) result in the use of back-off procedures similar to CSMA/CD(Ethernet) in order to reschedule a request.

TDD transition period 350—TDD transition period 350 separates the uplinkportion and the downlink portion and allows for transmitter (TX) toreceiver (RX) propagation delays for the maximum range of the cell linkand for delay associated with switching hardware operations from TX toRX or from RX to TX. The position of TDD transition period 350 may beadjusted, thereby modifying the relative sizes of the uplink portion andthe downlink portion to accommodate the asymmetry between data trafficin the uplink and the downlink.

A key aspect of the present invention is that the timing of the downlinkand uplink portions of each TDD frame must be precisely aligned in orderto avoid interference between sectors within the same cell and/or toavoid interference between cells. It is recalled from above that eachsector of a cell site is served by an individual RF modem in RF modemshelves 140A-140D and the internal RF modem shelves of central officefacilities 160A and 160B. Each RF modem uses an individual antenna totransmit and to receive in its assigned sector. The antennas fordifferent sectors in the same cell site are mounted on the same towerand are located only a few feet apart. If one RF modem (and antenna) aretransmitting in the downlink while another RF modem (and antenna) arereceiving in the uplink, the power of the downlink transmission willoverwhelm the downlink receiver.

Thus, to prevent interference between antennas in different sectors ofthe same cell site, the present invention uses a highly accuratedistributed timing architecture to align the start points of thedownlink transmissions. The present invention also determines the lengthof the longest downlink transmission and ensures that none of the uplinktransmissions begin, and none of the base station receivers begin toreceive, until after the longest downlink is completed.

Furthermore, the above-described interference between uplink anddownlink portions of TDD frames can also occur between different cellsites. To prevent interference between antennas in different cell sites,the present invention also uses the highly accurate distributed timingarchitecture to align the start points of the downlink transmissionsbetween cell sites. The present invention also determines the length ofthe longest downlink transmission among two or more cell sites andensures that none of the base station receivers in any of the cellsbegins to receive in the uplink until after the longest downlinktransmission is completed.

Within a cell site, a master interface control processor (ICP), asdescribed below in FIG. 4, may be used to align and allocate the uplinkand downlink portions of the TDD frames for all of the RF modems in anRF modem shelf. Between cell sites, the access processor may communicatewith several master ICPs to determine the longest downlink. The accessprocessor may then allocated the uplinks and downlinks across severalcell sites in order to minimize interference between cell sites and maydesignate on master ICP to control the timing of all of the master ICPs.

FIG. 4 illustrates the timing recovery and distribution circuitry inexemplary RF modem shelf 140 according to one embodiment of the presentinvention. RF modem shelf 140 comprises front panel interface 410 havingconnectors 411-414 for receiving input clock references and transmittingclock references. Exemplary connector 411 receives a first clock signalfrom a first external source (External Source A) and exemplary connector414 receives a second clock signal from a second external source(External Source B). Connector 412 outputs an internally generated clocksignal (Master Source Out) and connector 413 receives an external onesecond system clock signal (External 1 Second Clock).

RF modem shelf 140 also comprises a plurality of interface processor(ICP) cards, including exemplary ICP cards 450, 460, 470 and 480. ISPcard 450 is designated as a master ICP card and ICP card 480 isdesignated as a spare ICP card in case of a failure of master ICP card450. Within RF modem shelf 140, the ICP cards provide for controlfunctions, timing recovery and distribution, network interface, backhaulnetwork interface, protocol conversion, resource queue management, and aproxy manager for EMS for the shelf. The ICP cards are based on networkprocessor(s) that allow software upgrade of network interface protocols.The ICP cards may be reused for control and routing functions andprovide both timing and critical TDD coordinated burst timing for allthe RF modems in RF modem shelf 140 and for shelf-to-shelf timing forstacked frequency high density cell configurations.

The timing and distribution architecture in RF modem shelf 140 allowsfor three reference options:

Primary—An external input derived from another remote modem shelf actingas a master. BITS (Building Integrated Timing Supply) reference is asingle building master timing reference (e.g., External Source A,External Source B) that supplies DS1 and DS0 level timing throughout anoffice (e.g., 64K or 1.544/2.048 Mbps).

Secondary—A secondary reference may be derived from any designated inputport in RF Modem shelf 140. For remote RF modem shelf 140, this is oneof the backhaul I/O ports. An ICP card is configured to recover a timingsource and that source is placed on a backplane as a reference (i.e.,Network Reference (A/B)) to master ICP card 450.

Tertiary—An internal phase locked loop (PLL) may be used.

By default, two ICP cards are configured as a master ICP card and aspare ICP card. The active master ICP card distributes timing for all ofRF modem shelf 140. The timing distribution architecture of RF modemshelf 140 meets Stratum 3 levels of performance, namely a free-runaccuracy of +/−4.6 PPM (parts per million), a pull-in capability of 4.6PPM, and a holdover stability of less than 255 slips during the firstday.

There are three components to the timing distribution for the accessprocessor backplane:

1. Timing masters (ICP cards 450 and 480).

2. Timing slaves (ICP cards 460 and 470).

3. Timing references.

The timing masters are capable of sourcing all clocks and framingsignals necessary for the remaining cards within the AP backplane.Within a backplane, there are two timing masters (ICP cards 450 and480), which are constrained to the slots allocated as the primary andsecondary controllers. The timing masters utilize the redundant timingreferences (External Source A, External Source B, External 1 SecondClock) found on the backplane to maintain network-qualifiedsynchronization. ISP card 450 (and ISP card 480) comprises backhaulnetwork input/output (I/O) port 451, multiplexer 452 and PLL-clockgenerator 453. MUX 452 selects anyone of External Source A, ExternalSource B, Network Reference (A/B), and the signal from I/O port 451 tobe applied to PLL-clock generator 453. The timing master has missingclock detection logic that allows it to switch from one timing referenceto another in the event of a failure.

Timing is distributed across a redundant set of clock and framingsignals, designated Master Clock Bus in FIG. 4. Each timing master(i.e., ICP cards 450 and 480) is capable under software control ofdriving either of the two sets of clock and framing buses on thebackplane. Both sets of timing buses are edge-synchronous such thattiming slaves can interoperate while using either set of clocks.

The timing supplied by the timing master (e.g., ICP card 450) consistsof a 65.536 MHZ clock and an 8 KHz framing reference. There is a primaryand secondary version of each reference. To generate these references,the primary and secondary timing masters are provisioned to recover thetiming from one of the following sources:

Table of Clock Source Interface Definitions Source Frequency ExternalBITS (EXT REF A) 64K, 1544K, 2048K External BITS/GPS (EXT REF B) 64K,1544K, 2048K External GPS Sync. Pulse 1 second pulse On-Card ReferencePer I/O reference Network I/O Derived Reference A Per I/O referenceNetwork I/O Derived Reference B Per I/O reference

To simplify clock distribution and to provide redundancy all the clocksare derived from a common clock source. The following table summarizesthe backplane reference clocks as well as the clock rates of the variousbackplane resources and how they are derived from these references.

Table of Buses and Associated Clocks Clock Frequency Division or RatioCommon Reference 65.536 MHZ Not Applicable Clock Common Sync 1 Hz NotApplicable Pulse Framing Reference 8 KHz (125 usec) Free-run framingprovided by Primary or Secondary Clock Masters Referenced to CommonReference Clock Cell/Packet Clock 32.768 MHZ Reference Clock/2 Rate TDMBus Rate 8.192 MHZ Reference Clock/8 RF Reference 10.000 MHZ Free-run RFreference clock Clock Communications 100 MHZ Derived from free-run BusReference Clock High-speed 1.31072 GHz REF. Clock × 20 Serial LinksHigh-speed 2.62144 GHz REF. Clock × 40 Serial Links High-speed TBD REF.Clock × N Serial Links

Timing slaves (i.e., ICP cards 460 and 470) receive the timing providedby redundant sets of clock and framing buses. Under software control,timing slaves choose a default set of clocks from either the A-side orB-side timing buses. They also contain failure detection logic such thatclock and framing signal failures can be detected. Once a clock orframing failure is detected, the timing slave automatically switches tothe alternate set of timing buses. ICP cards 460 and 470 containbackhaul I/O ports 461 and 471, respectively, which may be used to bringin external timing signals from other RF modem shelves in the network.The timing masters (i.e., ICP cards 450 and 480), also contain thetiming slave function insofar as they also utilize the timing providedon the backplane clock and framing buses.

A qualified timing reference is required for the timing master to derivebackplane timing and to maintain synchronization within network 100 andwith any outside network. Under software control, an access processorcard can be assigned to derive this timing and to drive one of the twotiming reference buses. Ideally, a second, physically separate card willcontain a second qualified timing source and drive the second backplanetiming reference.

In the event that no qualified timing is present from trunk interfaces,the access processor backplane has connections which allow externalreference timing (e.g., a GPS-derived clock) from the interface tray tobe applied to the backplane. A one pulse-per-second (1PPS) signal isdistributed to all system cards for time stamping of system events anderrors. Installations involving multiple access processor shelvesrequire the timing reference to be distributed between all accessprocessor backplanes. In this scenario, the timing reference for a givenbackplane is cabled to the remaining backplanes through externalcabling. Multiple remote modem shelves are utilized to distributehigh-capacity backhaul traffic to one or more additional co-locatedmodem shelves. Traffic is distributed among the shelves through T1, T3,OC3 and/or other broadband telecommunication circuits. To maintainnetwork timing, the additional shelves are slaved to these distributionlinks and recover timing through the same PLL mechanisms as the head-endshelf.

FIG. 5A illustrates exemplary time division duplex (TDD) frame 500according to one embodiment of the present invention. FIG. 5Billustrates exemplary transmission burst 520 containing a single FECblock according to one embodiment of the present invention. FIG. 5Cillustrates exemplary transmission burst 530 containing multiple FECblocks according to one embodiment of the present invention.

TDD frame 500 comprises a downlink portion containing preamble field501, management field 502, and N modulation groups, including modulationgroup 503 (labeled Modulation Group 1), modulation group 504 (labeledModulation Group 2), and modulation group 505 (labeled Modulation GroupN). As explained above in FIG. 3, a modulation group is a group ofdownlink slots transmitted to one or more subscribers using a commonscheme of one or more of: 1) modulation format, 2) FEC codes, and 3)physical beam forming.

TDD frame 500 also comprises an uplink portion containingtransmitter-transmitter guard (TTG) slot 506, 0 to N registration (REG)minislots 506, 1 to N contention (CON) request minislots 508, Nsub-burst groups, including sub-burst group 509 (labeled Sub-Burst 1)and sub-burst group 510 (labeled Sub-Burst N), and receiver-transmitterguard (RTG) slot 511. As explained above in FIG. 3, a sub-burst group isa group of uplink slots transmitted to one or more subscribers using acommon scheme of one or more of: 1) modulation format, 2) FEC codes, and3) physical beam forming.

Each modulation group and each sub-burst group comprises one or moretransmission bursts. Exemplary transmission burst 520 may be used withina single modulation group in the downlink and covers one or moredownlink slots. Transmission burst 520 also may be used within a singlesub-burst group in the downlink and covers one or more uplink slots.Transmission burst 520 comprises physical media dependent (PMD) preamblefield 521, MAC header field 522, data packet data unit (PDU) field 523,and block character redundancy check (CRC) field 524. Transmission burst530 comprises physical media dependent (PMD) preamble field 531, MACheader field 532, data PDU field 533, block CRC field 534, data PDUfield 535, block CRC field 536.

The start of every frame includes a Start-Of-Frame (SOF) field and a PHYMedia Dependent Convergence (PMD) field. PMD preambles are used toassist in synchronization and time-frequency recovery at the receiver.The SOF field allows subscribers using fixed diversity to test receptionconditions of the two diversity antennas.

The SOF PMD field is 2^(N) symbols long (typically 16, 32, 64 symbolslong) and consists of pseudo-random noise (PN) code sequences, Franksequences, CAZAC sequences, or other low cross-correlation sequences,that are transmitted using BPSK or QPSK modulation. The SOF field isfollowed by downlink management messages broadcast from the base stationto all subscribers using the lowest modulation or FEC index andorthogonal expansion. Management messages are transmitted bothperiodically (N times per hyperframe) and as required to changeparameters or allocate parameters. Management messages include:

1. DownLink Map indicating the physical slot (PS) where downstreammodulation changes (transmitted every frame);

2. UpLink MAP indicating uplink subscriber access grants and associatedphysical slot start of the grant (transmitted when changed and at aminimum of one second hyperframe periods (shorter periods areoptional));

3. TDD frame and physical layer attributes (periodic at a minimum of onesecond hyperframe period); and

4. Other management messages including ACK, HACK, ARQ requests, and thelike (transmitted as required).

The downlink management messages are followed by multi-cast and uni-castbursts arranged in increasing modulation complexity order. The presentinvention introduces the term “modulation group” to define a set ofdownstream bursts with the same modulation and FEC protection. Asubscriber, continuously receives all the downstream data in the TDDframe downlink until the last symbol of the highest modulation groupsupported by the link is received. This allows a subscriber maximum timeto perform receive demodulation updates.

The downlink-to-uplink transition provides a guard time (TTG) to allowfor propagation delays for all the subscribers. The TTG position andduration is fully programmable and set by management physical layerattribute messages. The TTG is followed by a set of allocated contentionslots that are subdivided between acquisition uplink ranging mini-slotsand demand access request mini-slots. The Uplink MAP message establishesthe number and location of each type of slot. Ranging slots are used forboth initial uplink synchronization of subscribers performing net entryand for periodic update of synchronization of active subscribers.Contention slots provide a demand access request mechanism to establishsubscriber service for a single traffic service flow. As collisions arepossible, the subscriber uses random back-off, in integer TDD frameperiods and retries based on a time out for request of service.Contention slots use the lowest possible modulation, FEC, and orthogonalexpansion supported by the base station.

The contention slots are followed by individual subscriber transmissions(sub-bursts) that have been scheduled and allocated by the base stationin the uplink MAP. Each subscriber transmission burst is performed atthe maximum modulation, FEC, and orthogonal expansion supported by thesubscriber. Finally, the subscriber transmissions are followed by theuplink-to-downlink transition which provides a guard time (RTG) to allowfor propagation delays for all the subscribers. The RTG duration isfully programmable and set by management physical layer attributemessages.

In the downlink, the Physical Media Dependent (PMD) burstsynchronization is not used. The transmission burst begins with the MACheader and is followed by the packet data unit (PDU) and the associatedblock CRC field that protects both the PDU and the header. The PDU maybe a complete packet transmission or a fragment of a much largermessage. When a channel requires more robust FEC, the PDU may be brokeninto segments that are protected by separate FEC CRC fields. This avoidswasting bandwidth with additional MAC headers.

One significant difference between the uplink and the downlink is theaddition of the PMD preamble. The PMD preamble length and pattern can beprogrammed by transceiver base station 110. Like the SOF field at thebeginning of the TDD Frame, the preamble provides a synchronizationmethod for the base station receiver. Uplink registration and rangingpacket bursts use longer PMD preambles.

The medium access control (MAC) layer protocol is connection orientedand provides multiple connections of different quality of service (QoS)to each subscriber. The connections are established when a subscriber isinstalled and enters operation fixed wireless access network 100.Additional connections can be established and terminated with the basestation transceivers as subscriber requirements changes.

As an example, suppose a subscriber access device supports two voicechannels and a data channel. The quality of service (QoS) on both of thevoice channels and data can set based on the service structure set bythe wireless service provider. At installation, a subscriber may startwith two service connections: a toll quality voice channel and a mediumdata rate broadband data connection. At a later point in time, thesubscriber may order and upgrade service to two toll quality voicechannels and high speed data connection (a total of three connections).

The maintenance of connections varies based on the type of connectionestablished. T1 or fractional T1 service requires almost no maintenancedue to the periodic nature of transmissions. A TCP/IP connection oftenexperiences bursty on-demand communication that may be idle for longperiods of time. During those idle periods, periodic ranging andsynchronization of the subscriber is required.

In an exemplary embodiment of fixed wireless access network 100, eachsubscriber maintains a 64-bit EUI for globally unique addressingpurposes. This address uniquely defines the subscriber from within theset of all possible vendors and equipment types. This address is usedduring the registration process to establish the appropriate connectionsfor a subscriber. It is also used as part of the authentication processby which the transceiver base station and the subscriber each verify theidentity of the other.

In the exemplary embodiment, a connection may be identified by a 16-bitconnection identifier (CID) in MAC header 522 or MAC header 532. Everysubscriber must establish at least two connections in each direction(upstream and downstream) to enable communication with the base station.The basic CIDs, assigned to a subscriber at registration, are used bythe base station MAC layer and the subscriber MAC layer to exchange MACcontrol messages, provisioning and management information.

The connection ID can be considered a connection identifier even fornominally connectionless traffic like IP, since it serves as a pointerto destination and context information. The use of a 16-bit CID permitsa total of 64K connections within the sector.

In an exemplary embodiment of fixed wireless access network 100, the CIDmay be divided into 2 fields. Bits [16:x] may be used to uniquelyidentify a subscriber. In a cyclo-stationary receiver processing at abase station, this would set the antenna, equalizer, and other receiverparameters. Bits [x:1] may be used to indicate a connection to a type ofservice. Each subscriber service can have individual modulation format,FEC, and ARQ. Thus, within a single sub-burst group transmitted by asubscriber, the voice data may use one type of modulation format, FEC,and ARQ, and the broadband internet service may use a differentmodulation format, FEC, and ARQ. Similarly, within a single modulationgroup transmitted to the subscriber, the voice data may use one type ofmodulation format, FEC, and ARQ, and the broadband internet service mayuse a different modulation format, FEC, and ARQ.

As an example, bits [16:7] of the CID may identify 2̂10 or 1024) distinctsubscribers and bits [6:1] may identify. 2̂6=64 possible connections. Anapartment building could be given a set of subscriber ports [16:9] sothat bits [9:7] allow 28 connections or 256 connections.

Requests for transmission are based on these connection IDs, since theallowable bandwidth may differ for different connections, even withinthe same service type. For example, a subscriber unit serving multipletenants in an office building would make requests on behalf of all ofthem, though the contractual service limits and other connectionparameters may be different for each of them.

Many higher-layer sessions may operate over the same wireless connectionID. For example, many users within a company may be communicating withTCP/IP to different destinations, but since they all operate within thesame overall service parameters, all of their traffic is pooled forrequest/grant purposes. Since the original LAN source and destinationaddresses are encapsulated in the payload portion of the transmission,there is no problem in identifying different user sessions.

Fragmentation is the process by which a portion of a subscriber payload(uplink or downlink) is divided into two or more PDUs. Fragmentationallows efficient use of available bandwidth while maintaining the QoSrequirements of one or more of services used by the subscriber.Fragmentation may be initiated by a base station for a downlinkconnection or the subscriber access device for the uplink connection. Aconnection may be in only one fragmentation state at any given time. Theauthority to fragment data traffic on a connection is defined when theconnection is created.

The MAC layer protocol in wireless access network 100 also supportsconcatenation of multiple PDUs in a single transmission in both theuplink and the downlink, as shown in FIG. 5C. Since each PDU contains aMAC header with the CID, the receiving MAC layer can determine routingand processing by higher layer protocols. A base station MAC layercreates concatenated PDUs in the uplink MAP. Management, traffic data,and bandwidth may all be concatenated. This process occurs naturally inthe downlink. In the uplink, concatenation has the added benefit ofeliminating additional PMD preambles.

FIG. 6 depicts flow diagram 600, which illustrates the adaptivemodification of the uplink and downlink bandwidth in the air interfacein wireless access network 100 according to one embodiment of thepresent invention. Initially, an RF modem shelf, such as RF modem shelf140A, receives new access requests from subscriber access devices infixed wireless access network 100 and determines traffic requirementsfor each new and existing subscriber in each sector of a single cellsite (process step 605). The traffic requirements of each subscriber maybe established in a number of ways, including minimum QoS requirements,service level agreements, past usage, and current physical layerparameters, such as modulation index, FEC codes, antenna beam forming,and the like. The RF modem shelf then determines from the subscribertraffic requirements the longest downlink portion of an TDD frame ineach sector of a single cell site (process step 610).

Next, the access processor for the RF modem shelf (or the RF modem shelfitself) determines the appropriate allocation of downlink and uplinkportions of TDD frames for a single cell site in order to minimize oreliminate interference within the cell site (process step 615).Bandwidth is allocated, and TDD transition period 350 is positioned,such that the longest downlink transmission is complete before anyreceiver in the cell site starts to listen for the uplink transmission.

Next, if global allocation of downlink and uplink bandwidth acrossmultiple cell sites is being implemented (generally the case), theaccess processor determines the longest downlink portion of any TDDframe across several closely located cell sites stations (process step620). The access processor then determines the allocation of uplink anddownlink bandwidth for all TDD frames across several closely locatedcell sites in order to minimize or eliminate cell-to-cell interference(process step 625). Again, bandwidth is allocated, and TDD transitionperiod 350 is positioned, such that the longest downlink transmission iscomplete before any receiver in any of the closely located cell sitesstarts to listen for uplink transmissions. Finally, the downlinkportions of the TDD frames are launched simultaneously using the highlyaccurate clock from the distributed timing architecture (process step630).

The dynamic application of TDD bandwidth allocation is bounded by setminimum and maximum boundaries set by the service provider, based ontraffic and network analysis. Further, the bandwidth bounds may beallocated in sub-groupings based on established quality of service (QoS)requirements (e.g., voice data) and Service Level Agreements. (SLA)(e.g., broadband data rate) as the primary consideration and with bestefforts, non-QoS data, and IP traffic as secondary considerations. Thebandwidth bounds may be allocated based on the fact that a subscribermay support more that one interface and thus more than one modulationformat in order to achieve required error rates for one or more servicesprovided to the subscriber.

FIG. 7 depicts flow diagram 700, which illustrates the adaptiveassignment of selected link parameters, such as modulation format,forward error correction (FEC) codes, and antenna beam forming, to theuplink and downlink channels used by each subscriber in wireless accessnetwork 100 according to one embodiment of the present invention. The RFmodem shelf monitors data traffic between subscribers and base stationand determines for each subscriber the most efficient combination ofmodulation format, FEC code, and/or antenna beam forming for the uplinkand downlink.

The selected combination is based at least in part on the error ratesdetected by the RF modem shelf when monitoring the data traffic. If theerror rate for a particular subscriber is too high in either the uplinkor the downlink, the RF modem shelf can decrease the modulation formatcomplexity and use a higher level of FEC code protection in either theuplink or the downlink in order to reduce the error rate. Conversely, ifthe error rate for a particular subscriber is very low in either theuplink or the downlink, the RF modem shelf can increase the modulationformat complexity and use a lower level of FEC code protection in eitherthe uplink or the downlink in order to increase the spectral efficiency,provided the error, rate remains acceptably low. Different modulationformats and FEC codes may be used for different services (e.g., voice,data) used by a subscriber (process step 705).

Next, the RF modem shelf assigns subscribers to modulation groups in thedownlink and to sub-burst groups in the uplink (process step 710). Thebase station transceiver then transmits media access fields (e.g.,signaling, ACK & NACK) using the lowest modulation format/FEC codecomplexity. The base station transceiver then transmits the remainingmodulation groups in the downlink to the subscribers in increasing orderof modulation format/FEC code complexity (process step 715). When thedownlink is complete, the base station transceiver receives registration& contention minislots transmitted by the subscriber access devicesusing the lowest modulation format/FEC code complexity. The base stationtransceiver then receives the remaining sub-burst groups transmitted bythe subscribers in increasing order of modulation format/FEC codecomplexity (process step 720).

The use of adaptive link parameters improves the link throughput andcorrespondingly affects the bandwidth allocation described above in FIG.6. Link parameters apply not only to the transmitter but to the receiveras well. The present invention uses a bounded (finite) set of modulationformats to maximize bandwidth utilization to each subscriber in achannel or sector. In an exemplary embodiment of he present invention,the low complexity (low bandwidth efficiency) modulation formats usedfor media access fields (e.g., signaling, ACK, NACK) are binary phaseshift keying (BSPK or 2-PSK) and quadrature phase shift keying (QPSK or4-PSK). The present invention may also use multiple-code orthogonalexpansion codes in conjunction with the low complexity modulationformats for extremely robust communication. The higher complexity(higher efficiency) modulation formats used for the modulation groupsand the sub-burst groups may be 8-PSK, 16 quadrature amplitudemodulation (QAM), 32 QAM, 64 QAM, 128 QAM, and the like.

The present invention also uses bounded set of FEC codes to maximizebandwidth utilization to each subscriber in a channel or sector. Thelevel of FEC code protection is based on the services provided. Eachsubscriber may support multiple services.

In an advantageous embodiment of the present invention, the RF modemshelf may use packet fragmentation to transport data in either theuplink or the downlink. Fragmentation is the division of larger packetsinto smaller packets (fragments) combined with an ARQ (automatic requestfor retransmission) mechanism to retransmit and recover erroneousfragments. The RF modem shelf automatically reduces fragment size forhigh error rate channels. Fragmentation is applied for guaranteederror-free sources. The degree of fragmentation and ARQ is based on theservice provided, since each subscriber may support multiple services.

FIG. 8 depicts flow diagram 800, which illustrates the adaptiveassignment of selected link parameters to the different serviceconnections used by each subscriber in wireless access network 100according to one embodiment of the present invention. The RF modem shelfassigns connection identification (CID) values to the uplink and to theuplink connections used by a subscriber. If a subscriber uses more thanone service (e.g., two voice, One data), the RF modem shelf assignsseparate CID values to each uplink connection and separate CID values toeach downlink connection (process step 805). As noted above, the CIDcomprises a bit field with the uppermost bits identifying the subscriberand the lowermost bits identifying a specific connection to thesubscriber. While many sets of adaptive transmission and receptionparameters are possible, there are a finite number of combination thatmake logical sense. These combinations are grouped into physical layerusage codes that are broadcast to subscribers as part of the generalheader of TDD superframe or frame header on a periodic basis. Theseapply to both the base station transmissions and the subscribertransmissions.

The RF modem shelf monitors data traffic between subscriber and basestation and determines for each connection the most efficientcombination of modulation format, FEC code, and/or antenna beam formingfor the uplink and downlink (process step 810). The RF modem shelf thenassigns each subscriber connection to a modulation group in the downlinkand to a sub-burst group in the uplink (process step 815). The basestation transmits media access fields (e.g., signaling, ACK & NACK)using the lowest modulation format/FEC code complexity. Then basestation then transmits modulation groups to subscribers in increasingorder of modulation format/FEC code complexity (process step 820).Finally, the base station receives registration & contention minislotsusing the lowest modulation format/FEC code complexity. Then basestation then receives sub-burst groups from subscribers in increasingorder of modulation format/FEC code complexity (process step 825).

Physical layer usage codes are bound to subscriber CID values by aservice establishment protocol. If there is a degradation or improvementin the channel between a subscriber and the base station, a protocolexists so the subscriber access device and the base station may revisethe physical layer usage code and subscriber CID code. The codes andbindings can be added and deleted based on services requirements of thesubscriber.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form.

1. A system for communicating with a plurality of wireless accessdevices, comprising: at least one modem that communicates with theplurality of wireless access devices using time division duplex (TDO)frames, each TDD frame having an uplink portion for receiving data and adownlink portion for transmitting data; and at least one controllerassociated with the at least one modem that assigns each of theplurality of wireless access devices to at least one of a plurality ofmodulation groups, determines a modulation configuration for each of theplurality of modulation groups, and causes the modem to transmitdownlink data to wireless access devices assigned to a first of theplurality of modulation groups using a first modulation configurationand transmit downlink data to wireless access devices assigned to asecond of the plurality of modulation groups using a second modulationconfiguration, wherein at least one of the plurality of modulationgroups has at least two of the plurality of wireless access devicesassigned to it, and wherein the first modulation configuration isdifferent from the second modulation configuration.
 2. A system forcommunicating with a plurality of wireless access devices, comprising:at least one modem that communicates with the plurality of wirelessaccess devices using time division duplex (TDD) frames, each TDD framehaving an uplink portion for receiving data and a downlink portion fortransmitting data; and at least one controller associated with the atleast one modem that: determines a first combination of a modulationconfiguration and a beam forming to be applied to downlink datatransmitted to a first of the plurality of wireless access devices;determines a second combination of a modulation configuration and a beamforming to be applied to downlink data transmitted to a second of theplurality of wireless access devices; causes the at least one modem totransmit downlink data to the first of the plurality of wireless accessdevices using the first combination; and causes the at least one modemto transmit downlink data to the second of the plurality of wirelessaccess devices using the second combination, wherein the firstcombination is different from the second combination.